1. Field of the Invention
The present invention relates to fuel burners and more particularly to an improved pulverized fuel burner for reducing the pressure loss through the burner nozzle by efficiently breaking up, deflecting, and dispersing fuel ropes, defined subsequently, and for reducing the formation of nitric oxides by improving the fuel/air mixture in the burner nozzle.
The relatively high pressure loss of the primary air through the burner nozzle is an economic concern, since it increases the operating costs of the fossil fuel fired steam generating unit. This increase in operating cost is usually charged against the initial cost of the plant during bid evaluation. For this reason, it is advantageous to reduce the pressure drop in the burner nozzle as much as possible, thus minimizing the power requirements of the primary air fan.
One of the primary causes of large pressure losses in the burner nozzle is related to dispersion of fuel roping. Fuel roping is the concentration of the pulverized fuel in a relatively small area of the fuel transport pipe. Fuel roping is caused by the centrifugal flow patterns established by elbows and pipe bends. Fuel roping is unavoidable since a transition must be made from a vertical pipe run to a horizontal pipe run at the burner level.
The pressure drop in normal fluid flow and pneumatic conveying of solids in a burner nozzle can be separated into at least four effective forces:
(1) The friction of the fluid against the pipe wall. PA1 (2) The inertia force acting on the fluid. PA1 (3) The inertia and gravity forces acting on the solids. PA1 (4) The aerodynamic drag force acting on the solids.
In addition, there is a pressure drop that should be taken into account; the pressure drop caused by areas of flow separation. It has been determined that in a burner nozzle venturi with particle flow, a large area of flow separation exists in the diverging outlet section, thereby increasing the pressure drop in the burner nozzle and the operating costs.
When fuel roping occurs air flow distribution has a secondary effect on particle distribution. Once a particle attains momentum in a certain direction, it will change its direction of travel primarily by being impacted with a solid surface. Therefore, drag forces between the air and solid particles are of secondary importance while the momentum (mass) of the particle is of primary importance.
It is apparent from the foregoing discussion that a reduction in the pressure drop through the burner nozzle can be accomplished by a reduction in any of the four forces that contribute to a pressure drop and an elimination of flow separation. However, any attempt to reduce pressure losses must ensure adequate air-fuel mixing in order to provide flame stability and meet acceptable low NOx standards.
One source of atmospheric pollution is the nitrogen oxides (NOx) present in the stack emission of fossil fuel fired steam generating units. Nitric oxide (NO) is an invisible, relatively harmless gas. However, as it passes through the vapor generator and comes into contact with oxygen, it reacts to form nitrogen dioxide (NO.sub.2) or other oxides of nitrogen collectively referred to as nitric oxides. Nitrogen dioxide is a yellow-brown gas which, in sufficient concentrations is toxic to animal and plant life. It is this gas which may create the visible haze at the stack discharge of a vapor generator.
Nitric oxide is formed as a result of the reaction of nitrogen and oxygen and may be thermal nitric oxide and/or fuel nitric oxide. The former occurs from the reaction of the nitrogen and oxygen contained in the air supplied for the combustion of a fossil fuel whereas the latter results from the reaction of the nitrogen contained in the fuel with the oxygen in the combustion air.
The rate at which thermal nitric oxide is formed is dependent upon any or a combination of the following variables; (1) flame temperature, (2) residence time of the combustion gases in the high temperature zone and (3) excess oxygen supply. The rate of formation of nitric oxide increases as flame temperature increases. However, the reaction takes time and a mixture of nitrogen and oxygen at a given temperature for a very short time may produce less nitric oxide than the same mixture at a lower temperature, but for a longer period of time. In vapor generators of the type hereunder discussion wherein the combustion of fuel and air may generate flame temperatures in the order of 3,700.degree. F., the time-temperature relationship governing the reaction is such that at flame temperatures below 2,900.degree. F. no appreciable nitric oxide (NO) is produced, whereas above 2,900.degree. F. the rate of reaction increases rapidly.
The rate at which fuel nitric oxide is formed is principally dependent on the oxygen supply in the ignition zone and no appreciable nitric oxide is produced under a reducing atmosphere; that is, a condition where the level of oxygen in the ignition zone is below that required for a complete burning of the fuel.
It is apparent from the foregoing discussion that the formation of thermal nitric oxide can be reduced by reducing flame temperatures in any degree and will be minimized with a flame temperature at or below 2,900.degree. F. and that the formation of fuel nitric oxide will be inhibited by reducing the rate of oxygen introduction to the flame, i.e., air/fuel mixing.
However, reductions in flame temperature and the mixing of air and fuel also tend to reduce flame stability. Flame stability is essential for safe, efficient operation. Therefore, flame stability becomes a limiting factor to NOx reductions achievable by flame temperature and mixing reductions.
A pulverized fuel requires more excess air for satisfactory combustion than other fuels such as gas or oil. One reason is the inherent maldistribution of the fuel both to individual burner pipes and to the fuel discharge nozzles. Normally complete combustion of a pulverized fuel requires at least 15% excess air. Proper fuel and air mixing will decrease the need for excess air, result in the reduction of nitric oxide formation, and provide flame stability.
2. Description of the Prior Art
In the past some burner nozzles included a venturi section which was meant to break up fuel roping and evenly disperse the pulverized fuel at the outlet end of the burner nozzle. However, any attempt to reduce the pressure drop resulted in an unacceptable increase in the formation of NOx and inadequate flame stability, due to the improper mixing of the fuel and air.
U.S. Pat. No. 3,788,796 (Krippene, et al) shows a pulverized fuel burner including a venturi section and a conical end-shaped rod member. The purpose of this combination is to vary the velocity of the coal-air mixture and to enhance the fuel-air distribution. This particular design is ineffective in reducing the pressure drop through the burner nozzle.